The present invention relates to pneumatically-assisted hydraulic spray guns, particularly for the coating industry. More narrowly, the invention relates to a pneumatically-assisted hydraulic spray gun having improvements for improving the coating finish on particular surfaces, and with respect to particular types of coating materials.
Pneumatically-assisted hydraulic spray guns were first disclosed in U.S. Pat. No. 3,843,052, issued on Oct. 22, 1974, to Cowan. The principles of operation of this new spray gun enabled a significant reduction in the hydraulic liquid pressures previously required for hydraulic spray guns. For example, hydraulic spraying pressures up to the range of about 4,000 pounds per square inch (psi) were required in earlier hydraulic spray guns in order to achieve an adequate quality of atomization wherein the spray gun could be used for spraying fine finishes on smooth surfaces. The invention of the '052 patent enabled the hydraulic pressures for such spraying to be reduced by a factor of two or three, while requiring only a nominal additional air pressure source, in the range of 10-20 psi. When the air pressure source was properly directed, through the use of air jet openings on the spray gun, to impinge slightly ahead of the region of atomization, the vast improvement in atomization quality at lower hydraulic pressures was achieved.
The principles of the '052 patent have been followed in a number of subsequent patents, including patents which have disclosed devices operating at still lower hydraulic pressures, but at increased air pressures, such as U.S. Pat. No. 5,064,119, issued Nov. 12, 1991. This latter patent utilizes the air-assist features to achieve satisfactory atomization for relatively low hydraulic liquid atomization pressures.
Recent advances in the development of coatings for improved finish quality have yielded coating materials which have significantly higher solid content than previously was available. These newer coatings are more difficult to atomize as a result of the increased solid content, and therefore spray guns of the foregoing types have been required to necessarily operate at higher pressures. For such coatings, it is not unusual to require hydraulic pressures in the range of 3,000-4,000 psi, together with the use of air-assistance in the range of 10-30 psi. Furthermore, recent advances have led to the use of new types of solvents in combination with coating materials; particularly, the use of water as a solvent has become popular because of its improved environmental effects. However, water-based coating materials tend to increase the problems noted in the following paragraph, leading to a reduction in quality of the coating finish.
It has been noted that air-assisted hydraulic spray guns do suffer from the problem of providing quality finishes in certain spraying environments. For example, the air-assisted hydraulic spray gun device has not proven to be an adequate solution for producing high quality smooth finishes on woodwork, as for example in the case of the need to produce high quality furniture finishes. The problem appears to be caused by air entrainment into the spray particles, which results in air bubbles being created on the surface of the workpiece, and when the coating on the workpiece dries the air bubbles leave an irregular film surface which is unsatisfactory for quality films on furniture. These dried air bubbles appear as voids in the finish of about 0.4 mil in size, which degrades the finish. This problem is particularly noticeable when water base coatings are applied to wood finishes.
It has been discovered that the replacement of pressurized air to the spray gun with pressurized carbon dioxide (CO.sub.2) does tend to eliminate the voids above noted. This is apparently a result of the fact that CO.sub.2 is much more soluble in water and other solvents than is air; and therefore the CO.sub.2 bubbles which might otherwise occur are absorbed into the film before the film coating dries on the article. However, a problem in using CO.sub.2 in such spraying applications is that CO.sub.2 is considerably more expensive than pressurized air, and the volumetric quantities required significantly increase the cost of spraying with this gas. Therefore, this solution becomes practical only if the volumetric quantities of CO.sub.2 can be significantly reduced in air-assisted hydraulic spraying equipment. A further problem exists in that there may still be some air entrainment into the atomized spray pattern even when CO.sub.2 is used as the atomizing agent, because the spray pattern itself creates a certain degree of air turbulence in the zone of atomization immediately in front of the spray nozzle of the spray gun. This zone of turbulence will cause some air entrainment in the atomized particles, wherein the air-entrained particles are still conveyed to the workpiece to produce the voids described hereinbefore. Thus, the use of CO.sub.2 as the atomizing agent reduces the surface voids but does not entirely eliminate them, yet suffers from the problem of the heavy cost of consumption of the CO.sub.2 gas.